Most of the oxygen you are breathing was made by minuscule algae and bacteria. These plants, known as phytoplankton, provide half of the food on which all the animals on this planet depend – and, alarmingly, some say their numbers are plummeting. Here are some of the most photogenic. Caroline Morley

These unicellular protists make up a large group including parasitic, bioluminescent, heterotropic and photosynthetic species. They have two flagella providing propulsion and some steering when they swim in their distinctive spiralling style. They have been found in the fossil record from the Palaeocene.